Quadrature modulation systems modulate a first source signal onto an in-phase component (I) and a second source signal onto a quadrature component (Q) of a carrier signal, wherein the quadrature component is 90 degrees out of phase with the in-phase component. Both components are superposed and sent through a real channel. The reverse process is performed in the receiver. The received signal is down-converted to recover the first and second source signals. The first and the second source signals may be independent analog signals or may derived from a sole digital signal that has been split up into a first and a second digital source signal on the transmission side and that may be recovered from merging the received first and second source signals on the receiver side.
Receiver architectures that utilize I/Q signal processing are vulnerable to mismatches (imbalance) between the I and the Q paths (channels). For example, a splitter unit configured to divide the incoming received signal equally between the I and Q paths may introduce phase and gain differences. Different signal delays in the two paths may cause an additional phase imbalance. A phase shifter which, from a local oscillator output, generates a quadrature phase signal may provide a differential phase which is not exactly 90 degrees. The I and Q channel mixers might have different conversion modes which may be frequency dependent. In addition, filters and amplifiers in the I and Q paths are typically not perfectly matched. These I/Q mismatches have detrimental effects on the receiver performance.
The present invention provides a compensation technique that delivers satisfying results even if the I/Q mismatch contains a frequency-dependent portion resulting from the application of non-causal signal condition functions, for example filters, in the in-phase and quadrature paths.